That Half-zip sweater you bought

What looks good under a half zip sweater? Do i have to wear some sort of collard shirt every time or can i make it look a little more casual with just a t-shirt underneath. I have this one sweater that is pretty tight on me, not excessively, but i'm debating wearing it without a shirt under it, and just wearing the sweater as the shirt... think that would look too .... not right?

I like those half-zips but i'm just running out of close to wear under them. Suggestions anyone?